Magnesia or Sorel cement has been known for about 100 years. It has the advantage over Portland cement in that it cures faster and sets harder. On the other hand magnesia cement is slightly water soluble, which results in a reduced water resistance for products made from magnesia cement. However, according to Norwegian Pat. No. NO-PS 151035 its water resistance can be greatly improved by the addition of 0.5-2.0% by weight of ethyl silicate.
Building materials in sheet or strip form made by hot pressing and curing of a mixture consisting mainly of wood fibers, inorganic fillers, finely divided light calcined magnesia (MgO) and an aqueous magnesium chloride solution, and a method for producing them is described in Norwegian Pat. No. NO-PS 141889. Such a building product according to Norwegian Pat. No. NO-PS 141889 is characterized by the cured mixture having:
(a) a weight ratio of MgO to wood fiber of 1:3-1:4, PA0 (b) a weight ratio of MgCl.sub.2 to wood fiber material of 1:9-1:12, PA0 (c) an amount of chemically inactive silica of 2-15% based on the wood fiber content and PA0 (d) possibly up to 5%, preferably 2-4% waterglass, based on the wood fiber content.
The procedure according to Norwegian Pat. No. NO-PS 141889 is characterized by 30-40 parts by weight finely divided light calcined magnesia being mixed with 3-4 times its weight of a wood fiber filler which consists mainly of splinters with a length of at the most 20 mm, 2-8 parts by weight of inactive silica, at least one aqueous liquid containing 9-12 parts by weight MgCl.sub.2 and 30-60 parts by weight of water, with possibly 1-5 parts waterglass, and where this mixture is formed and cured from 3-20 minutes at a pressure of 15-50 bar and a temperature of 120.degree.-220.degree. C. Production of wood fiber boards according to Norwegian Pat. No. NO-PS 141889 can be continuous by extrusion or piece by piece in a flat press.
Wood fiber boards made according to Norwegian Pat. No. NO-PS 141889 are claimed to be dimensionally stble and "showed, even after long submersion in water no noticeable swelling" (Norwegian Pat. No. NO-PS 141189, page 7, lines 1-4).